Studies conducted with matrix metalloproteinase-3 (MMP-3; stromelysin-1) gene-deleted (MMP-3- /-) animals have indicated an important pro-inflammatory role for this enzyme in acute lung injury, but the mechanism by which stromelysin-1 contributes to the disease process is not well understood. MMP-3 may play a direct role in damage to the alveolar wall. Damage to the alveolar wall, in and of itself, may be sufficient to facilitate lung injury. Alternatively, MMP-3 may promote lung injury by contributing to the generation of factors that stimulate neutrophil recruitment to the lung. Neutrophil chemotactic factors derived from non-collagenous components of the extracellular matrix as well as chemotactic cytokines elaborated by macrophages may be differentially elaborated in MMP-3 -/- animals as compared to normal controls. The overall goal of the proposed research is to evaluate the possible mechanisms in order to understand, specifically, how MMP-3 contributes to acute lung injury. In Specific Aim I, we will delineate the cellular sources of MMP-3 in the lungs of normal mice and determine how MMP-3 levels change during acute lung injury. In normal and MMP-3 -/- mice we will assess the production of several other MMPs that are known to play a role in inflammation and will concomitantly evaluate the production of MMP inhibitors under the same conditions. It is important to determine if there are compensatory changes in other MMPs or MMP inhibitors in animals lacking MMP-3. In specific Aims II and III we will utilize an in vitro model of an alveolar wall to directly assess the role of MMP-3 in damage to the alveolar wall and the role of MMP-3 in neutrophil migration across the alveolar wall. Studies will be conducted under conditions that lead to acute lung injury in intact animals and under conditions that result in neutrophil migration across the alveolar wall occurs but that do not lead to tissue damage, per se. Finally, in Specific Aim IV, we will assess the role of MMP-3 in alveolar wall damage in vivo under conditions that lead to acute lung inflammation or that lead to neutrophil influx into the alveolar space without tissue damage. These studies will provide an overall understanding of the mechanism(s) by which MMP-3 contributes to acute lung injury.